Internet Relay Chat (IRC) is an application layer protocol that facilitates the communication in the form of text. The chat process works on a client/server networking model. IRC clients are computer programs that a user can install on their system. These clients communicate with chat servers to transfer messages to other clients.[1] IRC is mainly designed for group communication in discussion forums, called channels,[2] but also allows one-on-one communication via private messages[3] as well as chat and data transfer,[4] including file sharing.[5]

Client software is available for every major operating system that supports Internet access.[6] As of April 2011, the top 100 IRC networks served more than half a million users at a time,[7] with hundreds of thousands of channels[7] operating on a total of roughly 1,500 servers[7] out of roughly 3,200 servers worldwide.[8]

Over the past decade IRC usage has been declining: since 2003 it has lost 60% of its users (from 1 million to about 400,000 in 2014) and half of its channels (from half a million in 2003).[9]

Xaric, a text-based IRC client, in use on Mac OS X. Shown are two IRC channels and a private conversation with the software author.

IRC is an open protocol that uses TCP[1] and, optionally, TLS. An IRC server can connect to other IRC servers to expand the IRC network.[14] Users access IRC networks by connecting a client to a server.[15] There are many client implementations, such as mIRC, HexChat and irssi, and server implementations, e.g. the original IRCd. Most IRC servers do not require users to register an account but a nick is required before being connected.[16]

The protocol specified that characters were 8-bit but did not specify the character encoding the text was supposed to use.[20] This can cause problems when users using different clients and/or different platforms want to converse.

All client-to-server IRC protocols in use today are descended from the protocol implemented in the irc2.4.0 version of the IRC2 server, and documented in RFC 1459. Since RFC 1459 was published, the new features in the irc2.10 implementation led to the publication of several revised protocol documents (RFC 2810, RFC 2811, RFC 2812 and RFC 2813); however, these protocol changes have not been widely adopted among other implementations.[citation needed]

Although many specifications on the IRC protocol have been published, there is no official specification, as the protocol remains dynamic. Virtually no clients and very few servers rely strictly on the above RFCs as a reference.[citation needed]

Microsoft made an extension for IRC in 1998 via the proprietary IRCX.[21] They later stopped distributing software supporting IRCX, instead developing the proprietary MSNP.

The standard structure of a network of IRC servers is a tree.[22] Messages are routed along only necessary branches of the tree but network state is sent to every server[23] and there is generally a high degree of implicit trust between servers. This architecture has a number of problems. A misbehaving or malicious server can cause major damage to the network[24] and any changes in structure, whether intentional or a result of conditions on the underlying network, require a net-split and net-join. This results in a lot of network traffic and spurious quit/join messages to users[25] and temporary loss of communication to users on the splitting servers. Adding a server to a large network means a large background bandwidth load on the network and a large memory load on the server. Once established however, each message to multiple recipients is delivered in a fashion similar to multicast, meaning each message travels a network link exactly once.[26] This is a strength in comparison to non-multicasting protocols such as Simple Mail Transfer Protocol (SMTP) or Extensible Messaging and Presence Protocol (XMPP).

An IRC daemon can also be used on a local area network (LAN). IRC can thus be used to facilitate communication between people within the local area network (internal communication).[27][28]

IRC has a line-based structure. Clients send single-line messages to the server,[29] receive replies to those messages[30] and receive copies of some messages sent by other clients. In most clients, users can enter commands by prefixing them with a '/'. Depending on the command, these may either be handled entirely by the client, or (generally for commands the client does not recognize) passed directly to the server, possibly with some modification.[citation needed]

Due to the nature of the protocol, automated systems cannot always correctly pair a sent command with its reply with full reliability and are subject to guessing.[31]

The basic means of communicating to a group of users in an established IRC session is through a channel.[32] Channels on a network can be displayed using the IRC command LIST,[33] which lists all currently available channels that do not have the modes +s or +p set, on that particular network.

Users can join a channel using the JOIN command,[34] in most clients available as /join #channelname. Messages sent to the joined channels are then relayed to all other users.[32]

Channels that are available across an entire IRC network are prefixed with a '#', while those local to a server use '&'.[35] Other less common channel types include '+' channels—'modeless' channels without operators —[36] and '!' channels, a form of timestamped channel on normally non-timestamped networks.[37]

Users and channels may have modes that are represented by single case-sensitive letters[38] and are set using the MODE command.[39] User modes and channel modes are separate and can use the same letter to mean different things (e.g. usermode "i" is invisible mode whilst channelmode "i" is invite only.[40]) Modes are usually set and unset using the mode command that takes a target (user or channel), a set of modes to set (+) or unset (-) and any parameters the modes need.

Some but not all channel modes take parameters and some channel modes apply to a user on a channel or add or remove a mask (e.g. a ban mask) from a list associated with the channel rather than applying to the channel as a whole.[41] Modes that apply to users on a channel have an associated symbol that is used to represent the mode in names replies[42] (sent to clients on first joining a channel[34] and use of the names command) and in many clients also used to represent it in the client's displayed list of users in a channel or to display an own indicator for a user's modes.

In order to correctly parse incoming mode messages and track channel state the client must know which mode is of which type and for the modes that apply to a user on a channel which symbol goes with which letter. In early implementations of IRC this had to be hard-coded in the client but there is now a de facto standard extension to the protocol called ISUPPORT that sends this information to the client at connect time using numeric 005.[43][44]

There is a small design fault in IRC regarding modes that apply to users on channels: the names message used to establish initial channel state can only send one such mode per user on the channel,[42] but multiple such modes can be set on a single user. For example, if a user holds both operator status (+o) and voice status (+v) on a channel, a new client will be unable to see the less mode with less priority (i.e. voice). Workarounds for this are possible on both the client and server side but none are widely implemented.

A Channel Operator is a client on an IRC channel that manages the channel. IRC Channel Operators can be easily seen by the "@" symbol prefixed to their name, or a Latin letter "+o"/"o". On most networks, an operator can:

There are also users who maintain elevated rights on their local server, or the entire network; these are called IRC operators,[50] sometimes shortened to IRCops or Opers (not to be confused with channel operators). As the implementation of the IRCd varies, so do the privileges of the IRC operator on the given IRCd. RFC 1459[50] claims that IRC operators are "a necessary evil" to keep clean state of the network, and as such they need to be able to disconnect and reconnect servers. Additionally, to prevent malicious users or even harmful automated programs from entering IRC, IRC operators are usually allowed to disconnect clients and completely ban IPs or complete subnets. Networks that carry services (Nickserv et al.) usually allow their IRC operators also to handle basic "Ownership" matters. Further privileged rights may include overriding channel bans (being able to join channels they would not be allowed to join, if they were not opered), being able to op themselves on channels where they would not be able without being opered, being auto-opped on channels always and so forth.

The format of a hostmask is nick!user@host. The hostmask looks similar to, but should not be confused with an e-mail address.

The nick part is the nickname chosen by the user and may be changed while connected. The user part is the username reported by ident on the client.[53] If ident is not available on the client, the username specified when the client connected is used after being prefixed with a tilde.[54]

The host part is the hostname the client is connecting from. If the IP address of the client cannot be resolved to a valid hostname by the server, it is used instead of the hostname.

Because of the privacy implications of exposing the IP address or hostname of a client, some IRC daemons also provide privacy features, such as InspIRCD or UnrealIRCD's "+x" mode. This hashes a client IP address or masks part of a client's hostname, making it unreadable to users other than IRCops. Users may also have the option of requesting a "virtual host" (or "vhost"), to be displayed in the hostmask to allow further anonymity. Some IRC networks such as Freenode use these as "cloaks" to indicate that a user is affiliated with a group or project.[55]

Issues in the original design of IRC were the amount of shared state data[56][57] being a limitation on its scalability,[58] the absence of unique user identifications leading to the nickname collision problem,[59] lack of protection from netsplits by means of cyclic routing,[60][61] the trade-off in scalability for the sake of real-time user presence information,[62] protocol weaknesses providing a platform for abuse,[63] no transparent and optimizable message passing,[64] and no encryption.[65] Some of these issues have been addressed in Modern IRC.

Because IRC connections are usually unencrypted and typically span long time periods, they are an attractive target for Dos/DDoS attackers and hackers. Because of this, careful security policy is necessary to ensure that an IRC network is not susceptible to an attack such as a takeover war. IRC networks may also K-line or G-line users or networks that have a harming effect.

A small number of IRC servers support SSL/TLS connections for security purposes. This helps stop the use of packet sniffer programs to obtain the passwords of IRC users, but has little use beyond this scope due to the public nature of IRC channels. SSL connections require both client and server support (that may require the user to install SSL binaries and IRC client specific patches or modules on their computers). Some networks also use SSL for server to server connections, and provide a special channel flag (such as +S) to only allow SSL-connected users on the channel, while disallowing operator identification in clear text, to better utilize the advantages that SSL provides.[66][67]

IRC served as an early laboratory for many kinds of Internet attacks, such as using fake ICMP unreachable messages to break TCP-based IRC connections (nuking) to annoy users or facilitate takeovers.

One of the most contentious technical issues surrounding IRC implementations, which survives to this day, is the merit of "Nick/Channel Delay" vs. "Timestamp" protocols. Both methods exist to solve the problem of denial-of-service attacks, but take very different approaches. The problem with the original IRC protocol as implemented was that when two servers split and rejoined, the two sides of the network would simply merge their channels. If a user could join on a "split" server, where a channel that existed on the other side of the network was empty, and gain operator status, they would become a channel operator of the "combined" channel after the netsplit ended; if a user took a nickname that existed on the other side of the network, the server would kill both users when rejoining (i.e., 'nick-collision'). This was often abused to "mass-kill" all users on a channel, thus creating "opless" channels where no operators were present to deal with abuse. Apart from causing problems within IRC, this encouraged people to conduct denial of service attacks against IRC servers in order to cause netsplits, which they would then abuse.

The nick/channel delay (abbreviated ND/CD) solution to this problem is very simple. After a user signs off and the nickname becomes available, or a channel ceases to exist because all its users parted (as often happens during a netsplit), the server will not allow any user to use that nickname or join that channel, until a certain period of time (the delay) has passed. The idea behind this is that even if a netsplit occurs, it is useless to an abuser because they cannot take the nickname or gain operator status on a channel, and thus no collision of a nickname or 'merging' of a channel can occur. To some extent, this inconveniences legitimate users, who might be forced to briefly use a different name after rejoining (appending an underscore is popular).

The alternative, the timestamp or TS protocol, takes a different approach. Every nickname and channel on the network is assigned a timestamp – the date and time when it was created. When a netsplit occurs, two users on each side are free to use the same nickname or channel, but when the two sides are joined, only one can survive. In the case of nicknames, the newer user, according to their TS, is killed; when a channel collides, the members (users on the channel) are merged, but the channel operators on the "losing" side of the split lose their channel operator status.

TS is a much more complicated protocol than ND/CD, both in design and implementation, and despite having gone through several revisions, some implementations still have problems with "desyncs" (where two servers on the same network disagree about the current state of the network), and allowing too much leniency in what was allowed by the 'losing' side. Under the original TS protocols, for example, there was no protection against users setting bans or other modes in the losing channel that would then be merged when the split rejoined, even though the users who had set those modes lost their channel operator status. Some modern TS-based IRC servers have also incorporated some form of ND and/or CD in addition to timestamping in an attempt to further curb abuse.

Most networks today use the timestamping approach. The timestamp versus ND/CD disagreements caused several servers to split away from EFnet and form the newer IRCnet. After the split, EFnet moved to a TS protocol, while IRCnet used ND/CD.

In recent versions of the IRCnet ircd, as well as ircds using the TS6 protocol (including Charybdis), ND has been extended/replaced by a mechanism called SAVE. This mechanism assigns every client a UID upon connecting to an IRC server. This ID starts with a number, which is forbidden in nicks (although some ircds, namely IRCnet and InspIRCd, allow clients to switch to their own UID as the nickname).

If two clients with the same nickname join from different sides of a netsplit ("nick collision"), the first server to see this collision will force both clients to change their nick to their UID, thus saving both clients from being disconnected. On IRCnet, the nickname will also be locked for some time (ND) to prevent both clients from changing back to the original nickname, thus colliding again.

The first IRC server, tolsun.oulu.fi, a Sun-3 server on display near the University of Oulu computer centre. (2001)

There are thousands of running IRC networks in the world. They run various implementations of IRC servers, and are administered by various groups of IRC operators, but the protocol exposed to IRC users is very similar, and all IRC networks can be accessed by the same client software, although there might be slight incompatibilities and limited functionality due to the differing server software implementations.

The largest IRC networks have traditionally been grouped as the "Big Four"[68][69][70][71]— a designation for networks that top the statistics. The Big Four networks change periodically, but due to the community nature of IRC there are a large number of other networks for users to choose from.

There are three recognized URI schemes for Internet Relay Chat, irc, irc6, and ircs,[73] that (when supported) allows hyperlinks of various forms, including

irc://<host>[:<port>]/[<channel>[?<channel_keyword>]]

(where items enclosed within brackets ([,]) are optional) to be used to (if necessary) connect to the specified host (or network, if known to the IRC client) and join the specified channel.[74] (This can be used within the client itself, or from another application such as a Web browser). irc is the default URI, irc6 specifies a connection to be made using IPv6, and ircs specifies a secure connection.

Per the specification, the usual hash symbol (#) will be prepended to channel names that begin with an alphanumeric character—allowing it to be omitted. Some implementations (for example, mIRC) will do so unconditionally resulting in a (usually unintended) extra (for example, ##channel), if included in the URL.

Some implementations allow multiple channels to be specified, separated by commas.[citation needed]

Client software exists for various operating systems or software packages, as well as web-based or inside games. Many different clients are available for the various operating systems, including Windows, Unix & Linux, Mac OS X and mobile operating systems (such as iOS and Android). On Windows, mIRC is one of the most popular clients.[75]

Some programs which are extensible through plug-ins also serve as platforms for IRC clients. For instance, a client called ERC, written entirely in Emacs Lisp is included in v.22.3 of Emacs. Therefore, any platform that can run Emacs can run ERC.

A typical use of bots in IRC is to provide IRC services or a specific functionality within a channel such as to host a chat-based game or provide notifications of external events. However some IRC bots are used to launch malicious attacks such as denial of service, spamming, or exploitation. [83]

A program that runs as a daemon on a server and functions as a persistent proxy is known as a BNC or bouncer. The purpose is to maintain a connection to an IRC server, acting as a relay between the server and client, or simply to act as a proxy.[citation needed] Should the client lose network connectivity, the BNC may stay connected and archive all traffic for later delivery, allowing the user to resume his IRC session without disrupting their connection to the server.[84]

Furthermore, as a way of obtaining a bouncer-like effect, an IRC client (typically text-based, for example Irssi) may be run on an always-on server to which the user connects via ssh. This also allows devices that only have ssh functionality, but no actual IRC client installed themselves, to connect to the IRC, and it allows sharing of IRC sessions.[85]

To keep the IRC client from quitting when the ssh connection closes, the client can be run inside a terminal multiplexer such as GNU Screen or tmux, thus staying connected to the IRC network(s) constantly and able to log conversation in channels that the user is interested in, or to maintain a channel's presence on the network. Modelled after this setup, in 2004 an IRC client following the client-server model, called Smuxi, was launched.[86][87]

There are numerous search engines available to aid the user in finding what they are looking for on IRC.[88][89] Generally the search engine consists of two parts, a "back-end" (or "spider/crawler") and a front-end "search engine".

The back-end (spider/webcrawler) is the work horse of the search engine. It is responsible for crawling IRC servers to index the information being sent across them. The information that is indexed usually consists solely of channel text (text that is publicly displayed in public channels). The storage method is usually some sort of relational database, like MySQL or Oracle.[citation needed]

The front-end "search engine" is the user interface to the database. It supplies users with a way to search the database of indexed information to retrieve the data they are looking for. These front-end search engines can also be coded in numerous programming languages.

Most search engines have their own spider that is a single application responsible for crawling IRC and indexing data itself; however, others are "user based" indexers. The latter rely on users to install their "add-on" to their IRC client; the add-on is what sends the database the channel information of whatever channels the user happens to be on.[citation needed]

Many users have implemented their own ad hoc search engines using the logging features built into many IRC clients. These search engines are usually implemented as bots and dedicated to a particular channel or group of associated channels.

Extra modes: While the original IRC system used a set of standard user and channel modes, new servers add many new modes for such features as removing color codes from text, or obscuring a user's hostmask ("cloaking") to protect from denial-of-service attacks.[citation needed]

Proxy detection: Most modern servers support detection of users attempting to connect through an insecure (misconfigured or exploited) proxy server, which can then be denied a connection. An example is the Blitzed Open Proxy Monitor or BOPM. This proxy detection software is used by several networks, although that real time list of proxies is defunct since early 2006.[90]

Additional commands: New commands can be such things as shorthand commands to issue commands to Services, to network operator only commands to manipulate a user's hostmask.[citation needed]

Encryption: For the client-to-server leg of the connection SSL might be used (messages cease to be secure once they are relayed to other users on standard connections, but it makes eavesdropping on or wiretapping an individual's IRC sessions difficult). For client-to-client communication, SDCC (Secure DCC) can be used.[citation needed]

Connection protocol: IRC can be connected to via IPv4, the current standard version of the Internet Protocol, or by IPv6, the next-generation version of the protocol.

There is an effort of standardization and adding new features to the IRC protocol by IRCv3 working group.[91]

IRC still lacks a single globally accepted standard convention for how to transmit characters outside the 7-bit ASCII repertoire. IRC servers normally[clarification needed] transfer messages from a client to another client just as byte sequences, without any interpretation or recoding of characters. The IRC protocol (unlike e.g. MIME or HTTP) lacks mechanisms for announcing and negotiating character encoding options. This has put the responsibility for choosing the appropriate character codec on the client. In practice, IRC channels have largely used the same character encodings that were also used by operating systems (in particular Unix derivatives) in the respective language communities:

7-bit era: In the early days of IRC, especially among Scandinavian and Finnish language users, national variants of ISO 646 were the dominant character encodings. These encode non-ASCII characters like Ä Ö Å ä ö å at code positions 0x5B 0x5C 0x5D 0x7B 0x7C 0x7D (US-ASCII: [\]{|}). That is why these codes are always allowed in nicknames. According to RFC 1459, { | } in nicknames should be treated as lowercase equivalents of [ \ ] respectively.[20] By the late 1990s, the use of 7-bit encodings had disappeared in favour of ISO 8859-1, and such equivalence mappings were dropped from some IRC daemons.

8-bit era: Since the early 1990s, 8-bit encodings such as ISO 8859-1 have become commonly used for European languages. Russian users had a choice of KOI8-R, ISO 8859-5[citation needed] and CP1251, and since about 2000, modern Russian IRC networks convert between these different commonly used encodings of the Cyrillic script.

Multi-byte era: For a long time, East Asian IRC channels with ideographic scripts in China, Japan, and Korea have been using multi-byte encodings such as EUC or ISO-2022-JP. With the common migration from ISO 8859 to UTF-8 on Linux and Unix platforms since about 2002, UTF-8 has become an increasingly popular substitute for many of the previously used 8-bit encodings in European channels. Some IRC clients are now capable of reading messages both in ISO 8859-1 or UTF-8 in the same channel, heuristically autodetecting which encoding is used. The shift to UTF-8 began in particular on Finnish-speaking IRC (Merkistö(Finnish)).

Today, the UTF-8 encoding of Unicode/ISO 10646 would be the most likely contender for a single future standard character encoding for all IRC communication, if such standard ever relaxed the 510 bytes message size restriction. UTF-8 is ASCII compatible and covers the superset of all other commonly used coded character set standards.

Much like conventional P2P file sharing, users can create file servers that allow them to share files with each other by using customised IRC bots or scripts for their IRC client. Often users will group together to distribute warez via a network of IRC bots.[92]

Technically, IRC provides no file transfer mechanisms itself; file sharing is implemented by IRC clients, typically using the Direct Client-to-Client (DCC) protocol, in which file transfers are negotiated through the exchange of private messages between clients. The vast majority of IRC clients feature support for DCC file transfers, hence the view that file sharing is an integral feature of IRC.[93] The commonplace usage of this protocol, however, sometimes also causes DCC spam. DCC commands have also been used to exploit vulnerable clients into performing an action such as disconnecting from the server or exiting the client.

^ abCharalabidis, Alex (1999-12-15). "IRCing On The Macintosh: Ircle". The Book of IRC: The Ultimate Guide to Internet Relay Chat (1st ed.). San Francisco, California: No Starch Press. p. 61. ISBN1-886411-29-8. On large networks such as the Big Four— EFnet, IRCnet, Undernet, and DALnet— trying to list the thousands of channels with Ircle always causes you to disconnect due to the flood of information, while other clients can usually manage the feat, if you are on a direct Ethernet connection.

^ abRittner, Don (1999-03-03). The iMac Book (1st ed.). Scottsdale, Arizona: Coriolis Group. p. 215. ISBN1-57610-429-X. There are several large networks: EFnet, UnderNET, DALnet, and IRCnet make up the Big Four.

^"Blitzed Open Proxy Monitor Shuts Down". The Open Proxy Monitor which has been provided by the Blitzed IRC network has been shut down...The database was so large that it is near to impossible for the team to backup, or find a new location to continue the service. Added to that, most of the team members do not posses the time anymore to keep the service running.